For example, a liquid crystal display device includes a thin film transistor (TFT) substrate on which a plurality of TFTs and pixel electrodes connected thereto are arranged in a matrix, an opposite substrate which is disposed facing this TFT substrate and on which a color filter, a common electrode, and the like are formed, and a liquid crystal layer provided between the opposite substrate and the TFT substrate. A backlight which constitutes the light source is provided on the TFT substrate on the side opposite from the liquid crystal layer. A glass substrate is preferably used as the TFT substrate.
A chemical mechanical polishing (CMP) method has been known as a method for flattening the substrate surface. On a large glass substrate, however, it is difficult to flatten the entire surface with a high degree of accuracy using a CMP method. Therefore, an attempt has been made to perform polycrystallization of amorphous silicon or the like formed on the glass substrate by means of laser annealing and to flatten protrusions or the like at the crystal grain boundary.
Furthermore, in order to stabilize the characteristics of the TFTs formed on the glass substrate, blocking light from a backlight by forming a back-gate layer or light-blocking film (Patent Document 1) has been known.
Meanwhile, as shown in FIG. 15, which is a sectional view, Patent Document 2 discloses that a light-blocking film 102 is formed in advance on a transparent support substrate 101, an insulation layer 103 covering the light-blocking film 102 is formed and flattened, and the surface of an embedded oxide film 105 formed on a monocrystalline silicon substrate 104 is affixed to the surface of this insulation layer 103.
Moreover, Patent Document 3 discloses the following. Namely, recessed and protruding portions are formed on the surface of a semiconductor substrate, and after an insulation layer is formed thereon, an opening for forming a back-gate electrode is formed in a specified region of the insulation layer over the protruding portion, and a back-gate insulation film and a conductive material layer are then formed on the entire surface including the interior of the opening, after which a back-gate electrode is formed inside the opening by polishing the conductive material layer. Subsequently, an interlayer film is formed on the entire surface, the semiconductor substrate and a support substrate are affixed together with the interlayer film being interposed, and the semiconductor substrate is polished from the back surface and flattened so as to expose the insulation layer at the bottom of the recessed portion formed in the surface of the semiconductor substrate. By doing so, an attempt was made to produce SOI-type semiconductor devices having a back-gate electrode at a low cost using a substrate affixing method.